The removal of oxygen from inert gas streams such as impure argon or impure nitrogen is impractical when exceedingly low concentrations of oxygen are present in such inert gases. Conventional purification or separation techniques such as cryogenic distillation, pressure swing adsorption and vacuum swing adsorption are impractical for the removal of minor or trace quantities or oxygen from a major gas stream. The commercial removal of oxygen in minor quantities in inert gases, such as argon, face severe limitations, such as unfavorable economies of scale, limiting physical laws of thermodynamic equilibria and the impaired reversibility of prior art chemical processes for extracting the trace quantities of oxygen from major gas streams.
For instance, the removal of trace oxygen by the Deoxo process can only be practically achieved on a large scale. The prior art Deoxo process involved the removal of oxygen from argon by the reaction of the oxygen with added hydrogen over a catalyst. The gas must then be dried of the resulting water and excess hydrogen must be separated from the product gas stream. Because of the difficulty and expense in performing these diverse operations, the Deoxo process has only been utilized in a centralized manner in which various sources of oxygen-containing inert gas or argon have been shipped to a central processing plant for later redistribution.
Cryogenic separation techniques are exceedingly unfavorable for the removal of trace amounts of oxygen from argon in light of the particularly low temperature necessary for such a separation, as well as the close boiling points of such gases as oxygen and argon. This is exemplified in the commercial environment wherein few air separation facilities provide substantially pure, oxygen-free, argon.
The prior art has utilized physical adsorbents, such as molecular sieve beds, for the removal of oxygen from inert streams, such as argon or nitrogen. However, the physical adsorption technique is dependent upon thermodynamic equilibria. This approach, therefore, is relevant only to mixed gases wherein high concentrations of the impurity gas, i.e. oxygen, exists and minor or trace amounts of impurity gases in the separated components can be tolerated.
The use of Salcomine, a dioxygen absorbent, for the production of oxygen from air is taught in U.S. Pat. No. 2,450,276. The patent describes a process in which heat and vacuum are necessary to desorb the product oxygen from the Salcomine absorbent. In U.S. Pat. No. 2,523,549, Salcomine is used to remove oxygen from a hydrocarbon stream. The Salcomine is utilized in beds for alternating absorption duty. The patent teaches that the beds should be operated at a controlled temperature and the beds are desorbed with the assistance of a hot purge gas at up to 300.degree. F.
In U.S. Pat. No. 2,810,454, oxygen is removed from an impure argon stream by adsorption on a molecular sieve bed. The adsorption bed is refrigerated during the adsorption cycle and is heated and subjected to a purge gas during the desorption cycle.
U.S. Pat. No. 4,011,306 discloses the use of fluoramine, a dioxygen absorbent, for the separation of a product oxygen from an air stream for use in airplanes and other similar utilities. The dioxygen absorbent bed is cooled during the absorption cycle and is heated up to 200.degree. F. during the desorption cycle. In addition, the cycle of absorption is intermittently interrupted to heat the absorption beds to a high 390.degree. F. temperature to volatilize inerts captured in the absorbent. A purge of air or nitrogen is utilized to remove the latter inerts.
In U.S. Pat. No. 4,239,509, a molecular sieve adsorbent is utilized to remove nitrogen and oxygen from an argon stream.
Additional patents of general interest to the subject invention include: U.S. Pat. No. 2,874,030; U.S. Pat. No. 2,909,410; U.S. Pat. No. 3,055,732; U.S. Pat. No. 3,361,531; U.S. Pat. No. 3,969,481; U.S. Pat. No. 3,986,849; U.S. Pat. No. 3,996,028; U.S. Pat. No. 4,025,605; U.S. Pat. No. 4,077,780; U.S. Pat. No. 4,194,892; U.S. Pat. No. 4,203,958; U.S. Pat. No. 4,234,322; and U.S. Pat. No. 4,299,719.
The prior art processes for removing minor or trace quantities of oxygen from impure inert gas streams, such as argon or nitrogen, via chemical complexing or absorption suffer from the problems of expensive, complex separation systems and energy intensive regulation by refrigeration or heating of the absorbent at various stages of the process cycle for removing oxygen from the inert gas streams. The advantage in the present invention is found in overcoming many of these drawbacks and in using the inherent energy in the absorption system to reduce the amount of energy input required for the removal of oxygen from mixed gas streams, such as an oxygen contaminated argon or nitrogen gas stream. In particular, the advantage of the present invention is lower energy requirements for rigorous removal of minor quantities of oxygen.